This invention relates to making thin strip and, more particularly, casting of thin strip by a twin roll caster.
In a twin roll caster, molten metal is introduced between a pair of laterally positioned casting rolls that are counter-rotated and cooled so that metal shells solidify on the moving roll surfaces and are brought together at a nip between them to produce a solidified strip product (delivered downwardly from the nip between the casting rolls). The term “nip” is used herein to refer to the general region where the casting rolls are closest together. The molten metal is delivered from a ladle into a smaller vessel or vessels from which the molten metal flows through a metal delivery nozzle or delivery nozzles positioned side by side (also called the “core nozzles”) and form a casting pool of molten metal supported on the casting surfaces of the casting rolls and extending the length of the nip. This casting pool is locally confined by side plates or dams held in sliding engagement adjacent end portions of the casting rolls to confine the casting pool against outflow.
More particularly, the metal delivery nozzles receive molten metal from a movable tundish and deliver the molten metal in the casting pool in a desired flow pattern. Various designs for delivery nozzles have been previously proposed involving a lower portion submerged in the casting pool during a casting campaign, and having side openings through which the molten metal is capable of flowing laterally outwardly into the casting pool toward the casting surfaces of the rolls. Examples of such metal delivery nozzles are disclosed in U.S. Pat. No. 6,012,508. In prior art metal delivery nozzles, there has been a tendency to produce thin cast strip with defects from uneven solidification of the molten metal.
To inhibit certain defects in the cast strip, the conditions of the molten metal in the casting pool, including temperature, composition and flow rate, have been controlled. Particularly, controlling the flow rate and molten metal temperature in the area near where the side dams, casting rolls and meniscus of the casting pool intersect (i.e. the “triple point” area or region) is important to improve thin strip quality.
The formation of solid pieces known as “skulls” occur in the casting pool in the vicinity of the confining side plates or dams. The rate of heat loss from the casting pool is higher near the side dams in the “triple point region” due to conductive heat transfer through the side dams to the casting rolls. This localized heat loss near the side dams has a tendency to form skulls of solid metal in that region, which can grow to a considerable size and fall between the casting rolls causing defects in the cast strip. An increased flow of molten metal to these triple point regions near the side dams have been provided. See, U.S. Pat. No. 4,694,887 and U.S. Pat. No. 5,221,511. Increased heat input to these triple point regions has reduced formation of skulls.
To control flow in the triple point region, the distance between the side dams and the ends of the delivery nozzles near the side dams should be controlled and maintained during casting. This distance has been found so sensitive that even compensation for wear of the side dams needs to addressed. The approach in the past has been to provide a common support for each side dam and adjacent end portion of the delivery nozzle. In the past, coupling of the positioning and support for the delivery nozzles and side dams enabled control of the distance between the side dams and end portions of a delivery nozzle to improve the strip quality.
Such apparatus and methods for controlling the distance between the outer end portions of the delivery nozzles and the side dams during a campaign are disclosed in U.S. Pat. Nos. 6,910,523, 6,588,492, and 7,147,035. The apparatus and method disclosed have a carriage assembly to commonly supporting the side dams and end portions of the delivery nozzles and maintain distance between the side dams and end portions of the delivery nozzles with wear of the side dams. This common support maintains the distance between the side dam and end of the delivery nozzle. The delivery nozzles could be moved relative to the side dams by the carriage assembly during casting; however, the movement also involved simultaneously moving of both delivery nozzle and the adjacent side dam. This movement affects the side dam force and, thus, side dam wear. Moreover, the movement of the side dam by the support to compensate for wear of the side dam required repositioning of the delivery nozzle to maintain the distance between the side dam and the end portion of the near delivery nozzle.
Additionally, repositioning side by side delivery nozzles during the casting campaign as the side dam wears involves providing in a large gap between the refractory pieces of the main portion of the delivery nozzle in the center of the casting pool at the start of the casting campaign and a small gap between these refractory pieces of the main portion of the delivery nozzle at the end of the casting campaign. That in turn caused a ridge in the thickness at the center of the cast strip at the beginning of the cast and a dip in the thickness at the center of the cast strip toward the end of the cast, respectively.
We have found that quality of thin strip casting, particularly with control of skulls in the triple point region can be improved by entirely different approach with a delivery nozzle in which the refractory delivery end portions are separated from the main portion during the casting campaign. Having refractory delivery end portions and main portion separately supported during the casting campaign allows for control and maintenance of a set distance between the refractory delivery end portions of the delivery nozzle and the side dams throughout a casting campaign, while also controlling and maintaining a set end to end distance between the refractory pieces of the main portion at the center of the delivery nozzle.
Presently disclosed is a method for casting metal strip comprising:
(a) assembling a pair of casting rolls laterally disposed to form a nip between them, and adapted to support a casting pool of molten metal, with side dams positioned adjacent end portions of the casting rolls to confine the casting pool laterally;
(b) assembling an elongated metal delivery nozzle extending along and above the nip between the casting rolls, with a main portion comprising one or two refractory pieces with outlets adapted to deliver molten metal to the casting pool supported by the casting rolls, and refractory delivery end portions separately supported during casting adapted to move relative to the main portion at each end portion of the metal delivery nozzle, each refractory delivery end portion having a reservoir portion with passages there through adapted to deliver molten metal to the casting pool adjacent the side dams and the end portions of the casting rolls;
(c) providing a mechanism connected to each refractory delivery end portion adapted to move each refractory delivery end portion relative to the main portion as casting proceeds to maintain desired distance between the refractory delivery end portions and the side dams;
(d) delivering molten metal through the elongated metal delivery nozzle adapted to communicate through the outlets of the main portion with the casting pool of molten metal supported by the casting rolls, such that molten metal is caused to flow from the main portion through said outlets and through the reservoir portion passages of the refractory delivery end portions into the casting pool; and
(e) counter rotating the casting rolls to deliver cast strip downwardly from the nip as casting proceeds.
The method of casting metal strip may further comprise:
(f) connecting the refractory delivery end portions to the main portion during preheating; and
(g) separating the refractory delivery end portions from the main portion for casting.
In an embodiment, the method for casting metal strip may comprise a delivery nozzle with a main portion comprising two refractory pieces. The two refractory pieces may be positioned end-to-end. Each refractory piece of the main portion may have outlets adapted to deliver molten metal there through to the casting pool supported by the casting rolls. A mechanism is connected to each refractory piece that may be adapted to move each refractory piece relative to the other refractory piece. The refractory pieces may be positioned at a distance between 0.5 to 60 mm from each other. The distance between the refractory pieces may be adjusted during casting.
In another embodiment, the method for casting metal strip may comprise a metal delivery nozzle with a main portion comprising one refractory piece. The refractory delivery end portions may be positioned at each end of the main portion, but separately supported from the main portion during a casting campaign. Each refractory delivery end portion may be adapted to move relative to the main portion at each end portion of the metal delivery nozzle during casting. A mechanism is connected to each refractory delivery end portion that may be adapted to move said refractory delivery end portion relative to the main portion as casting proceeds to maintain desired distance between the respective refractory delivery end portions and the side dams.
The method for casting metal strip may further comprise positioning sensors to sense the positions of the refractory delivery end portions of the delivery nozzle and the side dams, and produce electrical signals indicative of said positions of the refractory delivery end portions of the delivery nozzle and the side dams positions. Additionally, the method for casting metal strip may further comprise controlling the positions of the refractory delivery end portions of the delivery nozzle and of the side dams responsive to said electrical signals produced by sensors so as to adjust the positions of the refractory delivery end portions of the delivery nozzle and of the side dams responsive to wear of said side dams.
The method for casting metal strip may further comprise positioning sensors to sense the positions of the refractory pieces of the main portion and of the refractory delivery end portions of the delivery nozzle, and produce electrical signals indicative of said positions. Additionally, the method for casting metal strip may further comprise controlling the positions of refractory piece or pieces of the main portion and of the refractory delivery end portions of the delivery nozzle responsive to said electrical signals produced by the sensors so as to adjust the positions of refractory pieces of the main portion and of the refractory delivery end portions relative to each other during casting.
Alternatively or in addition, the method for casting metal strip may further comprise positioning sensors to sense the positions of the refractory piece or pieces of the main portion, of the refractory delivery end portions of the delivery nozzle and of the side dams, and produce electrical signals indicative of said positions. Additionally, the method for casting metal strip may further comprise controlling the positions of the refractory piece or pieces of the main portion, of the refractory delivery end portions of the delivery nozzle and of the side dams responsive to said electrical signals produced by sensors so as to adjust the positions of the refractory delivery end portions of the delivery nozzle and of the side dams responsive to wear of said side dams.
Also disclosed is a metal delivery nozzle for a twin roll caster adapted to extend along and above a pair of casting rolls comprising:
a main portion comprising one or two refractory pieces with outlets adapted to deliver molten metal to a casting pool supported by the casting rolls during casting;
refractory delivery end portions separately supported adapted to move relative to the main portion at each end portion of the metal delivery nozzle, each refractory delivery end portion having a reservoir portion with passages there through adapted to deliver molten metal to the casting pool adjacent the side dams and the end portions of the casting rolls; and
a mechanism connected to each refractory delivery end portion adapted to move said refractory delivery end portion relative to the main portion as casting proceeds to maintain desired distance between the refractory delivery end portions and the side dams.
In an embodiment, the main portion may comprise two refractory pieces positioned end-to-end. Each refractory piece has outlets and may be adapted to deliver molten metal through said outlets to the casting pool supported by the casting rolls. A mechanism connected to each refractory piece may be provided to move each refractory piece relative to the other refractory piece. The refractory pieces of the main portion may be positioned at a distance between 0.5 to 60 mm from each other. The distance between the refractory pieces may vary during casting.
In another embodiment, the metal delivery nozzle has a main portion comprised of one refractory piece with outlets adapted to deliver molten metal to the casting pool supported by the casting rolls. A refractory delivery end portion may be located at each end of the main portion. Each refractory delivery end portion may be adapted to move relative to the main portion at each end portion of the metal delivery nozzle. Additionally, each delivery end portion may be connected to the main portion during preheating and separated from the main portion during casting.
The metal delivery nozzle for a twin roll caster adapted to extend along and above a pair of casting rolls may further comprise sensors to sense the positions of the refractory delivery end portions of the delivery nozzle and the side dams, and produce electrical signals indicative of said positions between the refractory delivery end portions of the delivery nozzle and the side dams. Additionally, the metal delivery nozzle for a twin roll caster adapted to extend along and above a pair of casting rolls may further comprise a control system adapted to control the positions of the refractory delivery end portions of the delivery nozzle and of the side dams responsive to said electrical signals produced by sensors so as to adjust the positions of the refractory delivery end portions of the delivery nozzle and of the side dams responsive to wear of the side dams.
Alternatively or in addition, the metal delivery nozzle for a twin roll caster adapted to extend along and above a pair of casting rolls may comprise sensors to sense the positions of the refractory piece or pieces of the main portion and of the refractory delivery end portions of the delivery nozzle, and produce electrical signals indicative of said positions. Additionally, the metal delivery nozzle for a twin roll caster adapted to extend along and above a pair of casting rolls may further comprise a control system adapted to control the positions of refractory piece or pieces of the main portion, of the refractory delivery end portions of the delivery nozzle and of the side dams responsive to said electrical signals produced by the sensors so as to adjust the positions of refractory piece or pieces of the main portion and of the refractory delivery end portions relative to each other.
In either embodiment, a mechanism may be connected to each refractory delivery end portion adapted to move said refractory delivery end portion relative to the main portion as casting proceeds to maintain a desired distance between the refractory delivery end portions and the side dams, the mechanism may be selected from the group consisting of servo-mechanisms, hydraulic mechanisms, pneumatic mechanisms, gear mechanisms, cog actuators, drive chain mechanisms, pulley and cable mechanisms, drive screw mechanisms, jack actuators, rack and pinion mechanisms, electro-mechanical actuators, electric motors, linear actuators, and rotating actuators.
Various aspects of this invention will become apparent from the following detailed description and accompanying drawings.